Integrated circuits (ICs), sometimes termed chips or dies, before ready for use, are typically packaged by encapsulation in a polymer material. The encapsulation process is to protect the individual IC during handling and operation.
In general, plastic encapsulation of ICs to form packaged ICs with electrical leads is as follows: ICs in die form are attached to mounting areas of a lead frame called islands, or die-attach pads. The lead flames are typically made of a thin, flat, metal sheet chosen for a number of characteristics including electrical conductivity. Lead frames typically have multiple, individual die attach pads, each for supporting an individual IC during encapsulation, wherein the individual dies are encapsulated in plastic material, leaving electrical leads protruding from the plastic encapsulation.
In the encapsulation process mating molds are placed against each side of the lead frame and liquid-phase polymer is injected to encapsulate IC dies attached to the die attach pads. The lead frame is designed to dam the flow of liquid-phase polymer as it moves to the outer edges of each individual mold cavity, stopping at the points where each mold contacts surfaces of the lead frame. To stop the flow of liquid-phase polymer between leads the lead frame has a pattern of dambars between individual leads, so that a contiguous band of material is formed around the periphery of the die attach pad. This contiguous band prevents the liquid polymer material from escaping from the mold cavity, and also allows the lead frame to remain one contiguous piece of material until subsequent trimming operations are performed.
After the polymer material solidifies and the molds are removed, a following operation in the manufacturing process removes the excess plastic in the region around the mold outline and also removes the dam bars, separating the leads electrically. The de-damming process removes the dambar portion of the lead frame material between each of the leads, providing electronic integrity for each lead. De-damming is a process of removing all or part of each dambar by use of a punch with a pattern of teeth conforming to the pattern of the dambars in the lead frame.
In a de-damming operation, a lead frame mounting the ICs to be trimmed is fed either manually or in an automated fashion through a punch press operation comprising at least one closable die with tooling for punching out the dambar material desired to be removed. Tooling within the die typically comprises an upper section holding punches having punch teeth arranged in a pattern to engage and remove the dambar portions of the leadframe, an intermediate portion adapted as a punch guide and stripper, and a lower section adapted as a guide for the punch teeth and a receptacle to receive dambar material trimmed and pushed by the punch teeth into the lower tooling area. In operation a lead frame is positioned on the lower tooling portion, the stripper portion closes on the top of the lead frame, and the punches are driven downward with the teeth passing through the lead frame, removing the dambar material, and pushing the removed material into the lower tooling section, where material is typically removed by vacuum apparatus. As teeth retract after a trimming operation, any material adhering to the punch teeth is typically removed (stripped) by the stripper/guide above the lead frame.
There are, as is widely known, many sorts of ICs having varying patterns of leads, and the lead frames used in the manufacturing process conform to the lead patterns of the ICs. There are, therefore, many different patterns of punches and teeth, and arrangement of punches in trimming tooling. These arrangements and variations are not particularly pertinent to the present invention, which may be practiced with essentially any known IC pattern in dambar removal.
There is a problem with conventional dambar removal tools. Apertures for punch teeth both in stripper/guides and in lower guide/receptacles, are subject to chipping and breaking as the punch teeth are repeatedly inserted and withdrawn. Also, continued use can stress or weaken the aperture pattern resulting in a higher probability of resultant breakage. Because these tools are typically fashioned from a single hardened tool blank, a broken tool cannot be easily repaired, and a new unit must be fabricated to replace a broken one. Precision grinding operations used in the fabrication of these tools are time consuming, delicate, and costly.
What is clearly needed is a method and apparatus whereby the delicate portion of the punch guide/receptacle can be replaced without re-manufacturing the entire tool. A method and apparatus such as this would save time and material, and make the maintenance process more manageable.